magnesium can help too.
I get those cramps every winter, pg or not, but they were a lot worse while I was pg, and during labour. trust me, you want to get it under control and not go through 7 hours of labour with cramps from toe to hip in both legs.
Get a gym instructor or physio to show you come calf and hamstring stretches that you can do too (most will suggest doing them in water when you are pg, like a hydrotherapy pool or similar - as long as the water temp is OK)